110 PALAEONTOLOGY OF THE UPPER MISSOURI. 



mella moderately thickened and flattened, its margin a little arched and smootli ; usually having a slight 

 umbilical groove along the anterior outer margin of the inner lip. Surface smooth, or only having obscure 

 lines of growth ; ornamented with alternate dark and light-colored zigzag bauds crossing the whorls at right 

 angles to the suture. 



Height, 0.40 inch ; greatest transverse diameter, measuring from the outer side of the aperture obliquely upwards 

 to the most prominent part of Ihe body whorl on the opposite side, 0.44 inch ; height of the aperture, 0.35 inch ; 

 breadth of do., 0.25 inch. 



This must have been a very pretty little shell before its bands of color were partly 

 obliterated by the fossilizing process. A few of the specimens, however, still retain 

 these colored markings so as to be quite distinctly seen when moistened. We know 

 of no fossil species with which it is liable to be confounded. Amongst recent 

 species, it is perhaps most nearly represented by the European N. rivalis, Parreyss, 

 which is of about the same size, and has much the same form and style of coloring, 

 though its aperture is rather less expanded, and its inner lip a little wider ; Avhile 

 its bands of color are narrower, and not so abruptly nor so frequently deflected. 



This species belongs to the subordinate group Neritina, as defined by H. & A. 

 Adams, and Chenu. 



Locality and position. — Head of Wind River, Dakota Territory, where it occurs 

 associated with Lioplacodes veternus, M. & H., in beds supposed to be of Jurassic 

 age. (Type No. 1979.) 



Order Cyclobraucliiata* 



? Family DENTALIIDiE. 



Shell tubular, slightly arched, truncated and open at both ends ; aper- 

 ture at tlie smaller end sometimes with a slit or lissure on the dorsal side. 



Operculum wanting. 



Animal with lingual membrane comparatively broad ; teeth in three 

 series, the middle denticulate, and the laterals broad and sim])le. Head 

 rudimentary, being without eyes or tentacles ; mouth with cirrhated lips. 

 Mantle circular, thick and fleshy in front, and covering the forepart of 

 the body. Gills two, symmetrical, and placed behind the heart. Foot 

 conical, small, or rudimentary, with two symmetrical side-lobes, and an 

 attenuated hollow base connecting with the stomach, which is provided 

 with a strong internal "gizzard." 



Although these curious mollusks are known to possess red blood like the earth- 

 worms, the rudimentary condition of their eyeless head, without traces of tentacles, 

 the position of their symmetrical gills, and the union of the sexes in each individual, 

 arc characters showing their low rank amongst the Gasteropoda. They are gene- 

 rally placed with the Prosohranchiata, though, as has been suggested by several 

 couchologists, they might Avitli almost equal propriety be ranged with the Oplstho- 

 bnmchiata. 



This family includes only the genera Dentalium, Entails, and Helonyx. 



